Processes · Programming in C

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Processes

Count the processes created by fork, understand what fork copies, write system and a small shell, reap your zombies, count signals. Chapter 6.

Programs are compiled with gcc -std=c11 -Wall -Wextra -Werror -D_POSIX_C_SOURCE=200809L.

Exercise · How many lines?

How many lines does each of these programs print? We only show the body of main; the #include <stdio.h> and <unistd.h> are implied. Draw the process tree.

Two forks in a row:

fork();
fork();
printf("x\n");

In a loop:

for (int i = 0; i < 3; i++)
    fork();
printf("x\n");

Only the child does it again:

if (fork() == 0)
    fork();
printf("x\n");

The child stops immediately:

for (int i = 0; i < 3; i++) {
    if (fork() == 0) {
        printf("fils %d\n", i);
        return 0;
    }
}
printf("père\n");

Exercise · What does fork copy?

#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

int main(void)
{
    int n = 1;
    if (fork() == 0) {
        n = 100;
        printf("fils : n = %d, &n = %p\n", n, (void *)&n);
        return 0;
    }
    wait(NULL);
    printf("père : n = %d, &n = %p\n", n, (void *)&n);
    return 0;
}

What does this program print? Are the two addresses the same?
How can the same address hold two different values?
We add printf("début ");, with no newline, just before the fork. What does the program print now?